OCR Text |
Show 148 DR. W. B. BEN H A M ON [Feb. 16, tinctly marked off from the clitellum, are more sharply separated from one another than is the case elsewhere in the body. The histological structure of the tubercula is represented in Plate VIII. fig. 14. On each side of somite xiii., between the outer and inner couples of chaetae, and rather nearer to the former, is a copulatory papilla, similar to those I have described above for M. papillata. The nephridiopores and the oviducal pores have the usual positions ; I could not detect any other apertures. The chaetae have the same arrangement and markings as in M. papillata ; they are longer than in that species and commence on the second somite. The annuli of the anterior somites are extremely well marked by deep grooves (Plate VII. fig. 6). The first somite is simple; the second is triannulated ; the third is biannulated ; somites iv. to ix. (inclusive) are triannulated, the third annulus in each case being very distinctly marked off; the tenth is biannulated ; the following ones are bi- or triannulated, though the aunuli are not so distinctly separated. Of the internal organs, one or two features are worthy of note. Three septa are stronger than the others ; the first lies between somites iv. and v., the second strong one lies in the middle of the somite viii., the third in the middle of somite ix. The oesophageal glands are nipped by the septum between somites ix. and x., as in M. beddardi. There is only one pair of ciliated rosettes (and probably, therefore, of testes) in somite x. and one pair of sperm-sacs in the same somite. In M. beddardi, also, there is but a single pair of each of these organs-the testes in somite x., the sperm-sacs in somite xi. I traced the sperm-duct as far as somite xvi., where it enters the muscles of the body-wall. But I have been unable to determine the position of the external aperture; for I did not wish to injure the single specimen. In M. rappi the spermducal pore lies in somite xx.; but I was unable to find the pore in M. beddardi, where, however, it has probably the same position1. The spermathecce are eight in number, arranged in couples on the anterior margin of each side of somites xii. and xiii.; they are larger than in M. papillata, though of the same shape. In number and position they agree with the condition in M. beddardi. The four species of Microchceta can thus be arrauged in two groups:- I. M. rappi aud M. papillata have two pairs of testes, and very numerous spermathecae arranged in rows of several in a row; the oesophageal glands lie wholly in one somite; the dorsal vessel is doubled in each of the somites v. to ix. II. M. beddardi and M. belli have but oae pair of testes ; have eight spermathecae arranged in couples ; the oesophageal glands are 1 In M. benhami this pore is between somites xx. and xxi. |